Brain stores of choline and acetylcholine were labeled by infusion of deuterated phosphorylcholine for various times and the percent of the incorporation determined. From this data the turnover rate of acetylcholine was calculated in stereomicroscopically dissected nuclei of rat brain. The results show that the concentration of acetylcholine in a given nucleus does not have any relationship with the dynamic state of the transmitter. Of the nuclei examined, the hippocampus has the largest fractional rate constant and the lowest concentration of acetylcholine. Moreover, caudate and accumbens have equal concentrations of acetylcholine but caudate has twice the turnover rate. Thus the study of the dynamic state may not be supplanted by measuring the acetylcholine content. The turnover rate following morphine administration was decreased in accumbens and hippocampus but not in septum, dorsal raphe and substantia nigra.